Study on Air Quality Index, Atmospheric Pollutants and Dry Deposition of PCDD/Fs in the Ambient Air near Southwest China

This study investigated the AQI (air quality index), atmospheric pollutants (PM 2.5 , NO 2 , and O 3 ), and dry deposition of PCDD/Fs in Chengdu and Chongqing, near southwest China from 2020 to 2021. The results showed that the implementation of strict epidemic prevention action led to a significant improvement in the air quality in 2020. However, the air quality index increased again in 2021 as the economic activity was recovered. In February 2020, at the height of the epidemic, the monthly average PM 2.5 and NO 2 concentrations were estimated as 50.9 µ g m –3 and 21.2 ppb, respectively in in Chengdu, and 48.9 µg m –3 and 22.9 ppb, respectively in Chongqing. In February 2021, when the economy was returned to normal, the monthly average PM 2.5 and NO 2 concentrations were 60.3 µ g m –3 and 36.2 ppb respectively in Chengdu and 52.4 µ g m –3 and 35.1 ppb, respectively in Chongqing. In addition, since O 3 concentrations were influenced by the VOCs–NO x ratio, the reduction of NO x emissions and the increase of VOC emissions during the epidemic control period caused a change in the VOCs–NO x ratio, and thus leading to an increase in O 3 concentrations. The monthly average concentration of O 3 from March 2021 to May 2021 was significantly lower than that of 2020. Furthermore, the dry deposition flux of PCDD/Fs also changed in the period before and after the epidemic. The dry deposition flux of PCDD/Fs in 2021 was significantly higher than that of 2020, which was associated with the fuel consumption of various industrial plants. This study indeed provided useful information for the contribution of scientific communities and important data bank for the future air quality control.


INTRODUCTION
Several cases of novel coronavirus pneumonia were reported in December 2019 due to the new pandemic (Sun et al., 2020). In order to curb the spread and deterioration of the epidemic, all provinces and municipalities across the country have launched a Level-1 response to major public health emergencies, taking unprecedentedly strict control measures such as road traffic control, restricting the residents from going out, closing the scenic spots and commercial areas, stopping market gatherings, suspending work and classes, and shutting down non-essential factories. These measures significantly caused a huge impact on political, economic, and social life. But on the other hand, there was the stagnation of various industries and sectors that led to a significant improvement in ambient air quality. The Ministry of Ecology and Environment (https://www.mee.gov.cn/) of China's ambient air quality statistics report showed that the average percentage of Class-I days in 337 prefecture-level-and-above cities nationwide increased

METHODS
The air quality was analyzed in two cities in southwest China: Chengdu (31°67ʹN, 104°06ʹE) and Chongqing (29°59ʹN, 106°54ʹE) from 2020 to 2021 as shown in Fig. 1. PM 2.5 , PM 10 , SO 2 , CO, NO 2 , and O 3 in these cities were investigated and integrated as AQI (http://www.tianqihoubao.com/lishi/). Chengdu, the capital of Sichuan Province, is an important central city in the western region with a resident population of 20,937,800. Chengdu is located in southwest China, the western part of the Sichuan Basin and the hinterland of the Chengdu Plain. It has a humid subtropical monsoon climate with abundant precipitation and 12 main streams such as the Min River and the Tuo River, as well as dozens of tributaries.
Chongqing, a municipality directly comes under the Central Government of China, is one of the important cities in China and the economic center of the upper reaches of the Yangtze River,

PCDD/F Concentration and Dry Deposition Flux of PCDD/Fs
The monthly PCDD/Fs concentration in Chengdu and Chongqing cities could be simulated by regression analysis Huang et al., 2011). In addition, to reduce the systematic error, two regression equations were used and the results were averaged from both. The two regression analysis equations are: Y 2 = 0.0117X -0.021 Y 1 , Y 2 : Total concentration of PCDD/Fs (pg m -3 ); X: PM 10 concentration in the urban ambient atmosphere (µg m -3 ).
The calculation method reported by Tian et al. (2021b) was used for the calculation of dry deposition flux of PCDD/Fs in this work, where the relevant parameters were obtained by using the methods reported in previous studies (Sheu et al., 1996;Shih et al., 2006).

AQI Distribution
The proportions of the six AQI classes in Chengdu and Chongqing in different seasons for 2020-2021 are shown in Figs. 2(a-d).
By comparing the AQI distribution in Chengdu for 2021 and 2020, it was found that the AQI distribution for both spring and summer in 2021 was basically the same as the distribution in 2020. However, the AQI distribution for the autumn and winter in 2021 was somewhat different from Fig. 2(a). The proportions of the six AQI classes for Chengdu in spring, summer, autumn, and winter in 2020. those in 2020. In the autumn of 2021, there were three consecutive days of severe pollution with AQI for class IV, but no such cases happened in the autumn of 2020. In addition, the proportion of AQI > 150 in winter 2021 (10.0%) is significantly higher than that in winter 2020 (4.4%). The AQI distribution for 2020 and 2021 in another important city in southwest China, Chongqing, is shown in Figs. 2(c) and 2(d).
The analysis of AQI in Chongqing for 2020 and 2021 showed that the AQI was significantly better in spring and summer than that of in autumn and winter. Furthermore, by comparing the AQI for both years, it was found that the AQI for spring and summer of 2021 (seasonal averaged values were 46.1 and 36.7) was better than that of spring and summer of 2020 (seasonally averaged values were 52.7 and 38.9). Conversely, the AQI in the autumn and winter of 2021 were higher than that of the autumn and winter of 2020. It was studied that the proportion of AQI > 100 was significantly higher in autumn and winter of 2021 (3.3% and 28.8%) than that of autumn and winter of 2020 (1.1% and 12.2%). By comparing the AQI distribution of Chengdu and Chongqing in 2020 and 2021, it was found that the air quality in Chongqing was better than that of in Chengdu. The seasonal averages of AQI in Chongqing in 2020 and 2021 were better than that of in Chengdu. Furthermore, Chongqing had AQI values below 100 in the spring of both 2021 and 2020, while Chengdu had AQI > 100 in the spring of both 2020 and 2021. In the winter of both 2020 and 2021, Chengdu had an AQI > 200 in some days, indicating a severe air pollution. However, Chongqing did not experience these phenomena in the winter. In addition, it was observed that the seasonal AQI levels in both cities were arranged in the order as follows: winter > spring > autumn > summer, suggesting that the air quality levels were significantly better in summer than that of in winter. The reason for this phenomenon could be attributed to the fact that Chengdu is located in the Sichuan basin, which is surrounded by mountains and has a high frequency of static winds throughout the year, and thus predisposing the city to severe air pollution events on a regional scale (Ning et al., 2018;Cao et al., 2020). Previous studies have also shown that the Sichuan basin could experience several severe haze events in winter, which were inextricably linked to the high intensity of local pollutant emissions, complex topography, and high population density (Wu et al., 2021).
Moreover, the comparison of data showed that both Chengdu and Chongqing had better AQI in the winter of 2020 than that of in the winter of 2021, due to the influence of the strict epidemic policy, which was consistent with the data published by the Chinese Ministry of Ecology and Environment. However, we would further explore the seasonal concentration changes of some common pollutants from 2020 to 2021, as well as the temporal concentration changes caused by the epidemic, and analyze the degree of contribution of each pollutant to the AQI.

PM 2.5 Concentration
Ambient fine particulate matter air pollution (PM 2.5 ) is generally a major risk factor for illness and death (Apte et al., 2015). The World Health Organization (WHO) has classified PM 2.5 as a Group 1 carcinogen in 2013 (WHO, 2017). Commonly, PM 2.5 comes from various industrial processes such as power generation, metallurgy, petroleum, chemistry, textile printing and dyeing, as well as smoke and dust emission during fuel combustion in the heating and cooking processes. In addition, exhaust emissions into the atmosphere from various types of transportation (using fuel) in the process of operation are also an important source of PM 2.5 .
The analysis of the monthly average PM 2.5 concentrations in Chengdu in 2020 and 2021 showed that the PM 2.5 concentrations in winter of 2021 (73.7, 60.3, and 64.4 µg m -3 in January, February and December, respectively) were significantly higher than that of the PM 2.5 concentrations in winter of 2020 (68.3, 50.9, and 62.3 µg m -3 , respectively). In other seasons, there was no much difference in the monthly average PM 2.5 concentrations in Chengdu both in 2020 and 2021, which indicated that PM 2.5 was an important influencing factor for the change of air quality in winter. In addition, during the calculation of AQI, it was found that PM 2.5 was the dominant pollutant which could lead to the higher AQI in winter, and its higher concentration was the main reason for the high incidence of respiratory diseases in winter (Sancini et al., 2014).
It was observed that the monthly average of PM 2.5 concentrations in both cities were lower in January and February 2020 than in 2021. This was mainly because of the reason that the winter of 2020 was the worst period of the epidemic, when a large number of factories were closed for holidays and residents largely did not leave their homes under a strict epidemic prevention policy. As a result, PM 2.5 pollution from factory emissions, from gasoline and from diesel transportation was greatly reduced (Rodríguez-Urrego et al., 2020;Chauhan and Singh, 2020). Due to the effective control of the epidemic by the government, the epidemic control policy was much more relaxed in 2021 when compared to 2020, so there was a significant increase in PM 2.5 monthly concentration in the winter of 2021.

NO 2 Concentration
NO 2 in the air is closely related to man-made pollution through the daily life activities such as factory production and road traffic, which mainly comes from industrial gas emissions, chemical fuel combustion, and vehicle exhaust from transportation (Li et al., 2015). It is a known fact that NO 2 pollution plays an important role in air pollution and NO 2 is not only the main primary pollutant, but also can be transformed into many secondary pollutants under photochemical reaction conditions (Liu et al., 2016). NO 2 has a strong irritating effect on the human respiratory mucosa, and its harmful effects on the lungs are significantly higher than that of those of SO 2 and NO, which would lead to emphysema and even death in serious cases (Lamsal et al., 2013;Ogen, 2020).
As shown in Fig. 4  From Fig. 4, it was obvious that both Chengdu and Chongqing showed a significant decrease in the monthly average of NO 2 concentration in February 2020. This was because of the reason that February 2020 was the most serious period of the epidemic, coinciding with the Chinese New Year holiday, when a large number of enterprises shut down their production, people across the country were isolated at home, and all kinds of transportation vehicles were restricted from traveling, which largely curbed the NO 2 emissions. During the epidemic, similar conditions were observed in several places around the world (Oo et al., 2021;Santoso et al., 2021). Then most plants started to resume work and production on a large scale in March and April 2020 when the worst of the epidemic was passed and so the monthly average concentration of NO 2 increased again.
In February 2021, the epidemic prevention measures were relatively relaxed, and hence the monthly average NO 2 concentration was increased significantly when compared with February 2020, which showed that the impact of the epidemic on the production and life of Chinese society was huge and was closely related to NO 2 emissions. In addition, the variation of NO 2 concentration was more influenced by the COVID-19 lockdown policy when compared to PM 2.5 . This result suggested that the sources of NO 2 emissions were mainly from various types of factories and motor vehicle exhaust. In contrast, PM 2.5 was still the main cause of winter air pollution in February 2020, although its concentration was decreased when compared to February 2021.
In addition, the analysis of the NO 2 concentrations distribution in Chengdu and Chongqing throughout the year revealed that the NO 2 concentration in summer was significantly lower than that of in winter. In winter, the operation of heating equipment caused more energy being consumed by combustion, and thus the thermal power generation as well as industrial fuel combustion have contributed more to NO 2 emissions than that of vehicle exhaust. Hence, this was the main reason for elevated NO 2 in winter.

O 3 Concentration
O 3 , a pale blue gas with a distinctive odor, is one of the major pollutants that produce photochemical smog and it is an important greenhouse gas (Bojkov, 1986;Worden et al., 2008). It was studied that it could have serious effects on climate, ecosystems, and the health of living organisms when the O 3 concentrations near the ground are too high (Nuvolone et al., 2018). In fact, O 3 exposure can cause damage to the respiratory and cardiovascular systems of sensitive populations (elderly or children). High levels of O 3 can even trigger the symptoms such as coughing and breathing difficulties and lead to an increased incidence of respiratory diseases such as asthma and chronic obstructive pulmonary disease (Yang et al., 2012;Bell et al., 2006).
As shown in Fig. 5(d), the daily ranges of O 3 concentrations in Chongqing from January to December 2021 were 7 and 37, 10 and 66, 13 and 59, 12 and 63, 16 and 100, 33 and 96, 27 and 115, 30 and 125, 33 and 79, 21 and 90, 9 and 42, and 8 and 37 ppb, and the averages were 18.8, 35.4, 32.2, 35.5, 48.3, 56.6, 64.4, 61.5, 56.6, 35.9, 19.8, and 19.7 ppb, respectively. he distribution of O 3 concentrations in Chengdu and Chongqing showed that the highest O 3 concentrations occurred from May to August, with a significant decrease in January, October, November and December. This was due to the reason that in the troposphere, the formation of O 3 was mainly due to the photochemical reactions of NO x . It was hypothesized that in the atmosphere, NO 2 would generate O 3 by photolysis reaction (λ ≤ 424 nm), and O 3 would further react with NO to form NO 2 and O 2 , and thus it would not lead to the accumulation of O 3 near the ground (Fishman and Seiler, 1983). However, in summer, however, normally the photochemical reactions would be dominated by intense UV light and high temperature, and thus facilitating the accumulation of O 3 near the ground . In addition, in the summer (sunny weather), in the strongest ultraviolet time mainly between 12:00 to 16:00, due to high temperature, low relative humidity, and weak wind transport capacity the ozone pollution would more likely to occur (Sartor et al., 1995). However, the photochemical reactions do not play a dominant role while the solar radiation ability is weak in winter, and hence the higher NO x concentration would promote the consumption of O 3 . So, the O 3 concentration would be significantly lower in winter than that of in summer.
Furthermore, it was known that the volatile organic compounds (VOCs) are also an important factor affecting the O 3 production. The VOCs will be oxidized to peroxy radicals (HO 2 and RO 2 ) when VOCs are present in the atmosphere, and these radicals will react competitively with O 3 , and thus disrupting the reaction balance between NO x and O 3 , which in turn will leads to the accumulation of O 3 and pollution (Wang et al., 2017). In previous studies it was also reported that the formation of O 3 depended on the ratio of VOCs-NO x (Pusede and Cohen, 2012). As it can be seen from the Fig. 5, the monthly average O 3 concentrations in Chengdu and Chongqing during the 2020 epidemic (March-June) were observed to be higher than that of those in 2021, and O 3 pollution was more severe particularly in Chengdu. From Fig. 4 it was observed that there was a significant reduction in the monthly average concentrations of NO 2 due to the epidemic lockdown policy, but however, this did not imply a reduction in O 3 generation. At the same time, due to strict quarantine measures of the epidemic, the long-term isolation of urban residents cooking at home could enhance the emissions of VOCs and CO, which caused the VOCs-NO x emissions ratio out of balance, and thus aggravating the O 3 pollution (Sicard et al., 2020). It was identified that the concentrations of O 3 showed a complex non-linear relationship with NO x and VOCs emissions and it was mainly dependent on the VOCs-NO x ratio. Hence, it was suggested that the multiple influencing factors must be considered for the treatment of O 3 pollution (Liu et al., 2021).
Analysis of the dry deposition distribution of PCDD/Fs in Chengdu and Chongqing revealed hat the dry deposition flux of PCDD/Fs generally showed a trend of the highest in winter and the lowest in summer, which is consistent with the monthly average concentration distribution of PM 2.5 and NO 2 . Similarly, the elevated dry deposition flux of PCDD/Fs in winter was caused by the cold weather leading to the opening of a large number of heating facilities and therefore there would be more fossil fuel combustion, leading to the increased particulate emissions (Cheruiyot et al., 2016). The dry deposition flux of PCDD/Fs was primarily from the particulate phase, and so the increase of particulate emissions inevitably led to an increase in dry deposition flux of PCDD/Fs (Yu et al., 2021). In addition, another trend was also observed: the dry deposition flux of PCDD/Fs in 2020 was significantly lower than that of in 2021 due to the strict epidemic control. This was because of the reason that a large number of industrial production activities, such as pharmaceutical industry, metal smelting, and waste-to-energy incineration, were forced to be affected during the outbreak closure, and thus reduced the pollutant emissions (Tian et al., 2021b;Liu et al., 2013). In addition, the quarantine of residents further reduced PCDD/F originating from exhaust emissions from vehicles. In 2021, the epidemic lockdown policy was lifted, a large number of factories resumed work and production, and urban life returned to normal, and so the dry deposition flux of PCDD/Fs was again increased in 2021 when compared to 2020.

CONCLUSION
1. Both Chongqing and Chengdu regions showed the better air quality in spring and summer than that of in autumn and winter both in 2020 and 2021. In addition, Chongqing had better air quality than Chengdu, which has repeatedly experienced heavy pollution with AQI > 200, due to its geographical location. The impact of the epidemic blockade led to the significant improvements in air quality in Chengdu and Chongqing in 2020. In the epidemic era, as the economy recovered and industries began to resume work and production, the air quality again declined, indicating the negative correlation of dependence between economic development and air quality.
2. Attributed to the epidemic quarantine measures, PM2.5 pollution from factory emissions, gasoline and diesel transportation was greatly reduced in January to March 2020, resulting into a significant reduction in PM 2.5 concentrations. As the economy recovered, the PM 2.5 concentrations was again rebounded in the post-epidemic era.
3. The steep drop in monthly average NO 2 concentrations in February 2020 significantly reflected the impact of the lockdown on air quality. The quarantine measures limited the use of the most fuel-fired transportation, and thus leading to a decrease in NO 2 concentrations. The rebounded NO 2 monthly average concentrations in 2021 clearly demonstrated that the social activities have returned to normal in the post-epidemic era and the economy was restarted and thus the air pollution was increased.
4. During the outbreak period of epidemic in 2020, O 3 concentrations increased rather than decreasing, and thus O 3 pollution was increased significantly. This anomaly was due to the complex non-linear relationship between O 3 concentration and VOCs-NO x emissions, and it was mainly influenced by the VOCs-NO x ratio. Although the NO x emissions were reduced, the large number of cooking activities during the isolation period increased the VOC emissions, and thus breaking the VOCs-NO x ratio, which in turn increased the O 3 production.
5. The PCDD/Fs dry deposition flux was also associated with control measures. This was due to the fact that a large number of industrial production activities such as pharmaceuticals, metal smelting and waste incineration were forced to be affected during the epidemic closure, and thus reduced the pollutant emissions. In 2021, the economic recovery will inevitably require the expansion of emissions from the above plants, leading to the increased dry deposition flux of PCDD/Fs again. 6. PM2.5, NO 2 and PCDD/Fs dry deposition flux showed a trend of the highest in winter and the lowest in summer, while ozone showed a trend of the highest in summer and the lowest in winter. In winter PM 2.5 and NO 2 were the main influencing factors for elevated AQI, while in summer the dominant factor for AQI was O 3 . Therefore, in order to improve the air quality, it should be focused on the seasonal characteristic pollutants for different seasons, and all kinds of complex human and meteorological factors should also be taken into the account.
7. The urban lockdown caused by the epidemic surprisingly resulted into a improvement in air quality, but however at the cost of a heavy blow to the economy only. Thus, it was suggested that in the post-epidemic era, the epidemic period could be considered or studied as a lesson to understand the relationship between the economic development and the air pollution and also to find a balance between the both.